Method of producing polarizing plate

ABSTRACT

There is provided a polarizing plate excellent in durability. A method of producing a polarizing plate according to an embodiment of the present invention includes: preparing a polarizing film laminate including a polarizer and a protective film arranged on at least one side of the polarizer; and shrinking the polarizing film laminate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. patent application Ser. No. 15/332,561,filed Oct. 24, 2016, which claims priority under 35 U.S.C. Section 119to Japanese Patent Application No. 2015-216398 filed on Nov. 4, 2015,the entire contents of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method of producing a polarizingplate.

2. Description of the Related Art

A polarizing plate has been used in an image display apparatus (e.g., aliquid crystal display apparatus) of a cellular phone, a notebookpersonal computer, or the like. In recent years, the use of thepolarizing plate in, for example, a meter display portion of anautomobile or a smart watch has been desired, and hence the formation ofthe polarizing plate into a shape other than a rectangular shape and theformation of a through-hole in the polarizing plate have been desired.However, when any such form is adopted, a problem in terms of durabilityis liable to occur. With a view to improving the durability, forexample, there has been proposed a method involving thermally treating apolarizer at a temperature of 95° C. or more, and laminating aprotective film on the thermally treated polarizer to provide apolarizing plate (see Japanese Patent Application Laid-open No. Hei 7(1995)-333425). However, a further improvement in durability has beenrequired.

SUMMARY OF THE INVENTION

The present invention has been made to solve the problem, and a primaryobject of the present invention is to provide a polarizing plateexcellent in durability.

A method of producing a polarizing plate according to an embodiment ofthe present invention includes: preparing a polarizing film laminateincluding a polarizer and a protective film arranged on at least oneside of the polarizer; and shrinking the polarizing film laminate.

In one embodiment of the present invention, the shrinking the polarizingfilm laminate is performed in a transmission axis direction of thepolarizer by 0.2% or more.

In one embodiment of the present invention, the method further includescutting the polarizing film laminate.

According to another aspect of the present invention, there is provideda polarizing plate. The polarizing plate is obtained by the productionmethod as described above.

According to the present invention, the polarizing plate excellent indurability can be obtained by shrinking the polarizing film laminateobtained by laminating the polarizer and the protective film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a polarizing film laminate according toone embodiment of the present invention.

FIG. 2 is a plan view of a polarizing plate according to one embodimentof the present invention.

FIG. 3A is a photograph for showing the periphery of a through-hole ofthe polarizing plate of Example 1 after a heat cycle test, FIG. 3B is aphotograph for showing the periphery of a through-hole of the polarizingplate of Example 2 after a heat cycle test, FIG. 3C is a photograph forshowing the periphery of a through-hole of the polarizing plate ofExample 3 after a heat cycle test, and FIG. 3D is a photograph forshowing the periphery of a through-hole of the polarizing plate ofComparative Example 1 after a heat cycle test.

FIG. 4A is a photograph for showing the state of the periphery of an endside of the polarizing plate along the transmission axis direction ofthe test sample of Example 1 after the heat cycle test, and FIG. 4B is aphotograph for showing the state of the periphery of an end side of thepolarizing plate along the absorption axis direction thereof.

FIG. 5A is a photograph for showing the state of the periphery of an endside of the polarizing plate along the transmission axis direction ofthe test sample of Comparative Example 1 after the heat cycle test, andFIG. 5B is a photograph for showing the state of the periphery of an endside of the polarizing plate along the absorption axis directionthereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention are described.However, the present invention is not limited to these embodiments.

A method of producing a polarizing plate according to an embodiment ofthe present invention includes: preparing a polarizing film laminateincluding a polarizer and a protective film arranged on at least oneside of the polarizer; and shrinking the polarizing film laminate.

A. Polarizing Film Laminate

FIG. 1 is a sectional view of a polarizing film laminate according toone embodiment of the present invention. A polarizing film laminate 10includes a polarizer 11, a first protective film 21 arranged on one sideof the polarizer 11, and a second protective film 22 arranged on theother side of the polarizer 11. The protective films 21 and 22 are eachtypically bonded to the surface of the polarizer 11 throughintermediation of an adhesive layer, though the layer is not shown.Although the protective films are arranged on both sides of thepolarizer in this illustrated example, a protective film may be arrangedonly on one side thereof.

A-1. Polarizer

The polarizer typically includes a resin film containing a dichromaticsubstance. Examples of the dichromatic substance include iodine and anorganic dye. The substances may be used alone or in combination. Ofthose, iodine is preferably used.

Any appropriate resin may be used as a resin for forming the resin film.A hydrophilic resin (e.g., a polyvinyl alcohol (PVA)-based resin) ispreferably used as the resin. Examples of the PVA-based resin includepolyvinyl alcohol and an ethylene-vinyl alcohol copolymer. The polyvinylalcohol is obtained by saponifying polyvinyl acetate. The ethylene-vinylalcohol copolymer is obtained by saponifying an ethylene-vinyl acetatecopolymer. The saponification degree of the PVA-based resin is typicallyfrom 85 mol % to 100 mol %, preferably 95.0 mol % or more, morepreferably 99.0 mol % or more, particularly preferably 99.93 mol % ormore. The saponification degree may be determined in conformity with JISK 6726-1994. The use of the PVA-based resin having such saponificationdegree can provide a polarizer excellent in durability.

The average polymerization degree of the PVA-based resin mayappropriately be selected depending on purposes. The averagepolymerization degree is typically from 1,000 to 10,000, preferably from1,200 to 6,000, more preferably from 2,000 to 5,000. The averagepolymerization degree may be determined in conformity with JIS K6726-1994.

The polarizer preferably shows absorption dichroism in the wavelengthrange of from 380 nm to 780 nm. The single axis transmittance (Ts) ofthe polarizer is preferably 40% or more, more preferably 41% or more,still more preferably 42% or more, particularly preferably 43% or more.A theoretical upper limit for the single axis transmittance is 50%, anda practical upper limit therefor is 46%. In addition, the single axistransmittance (Ts) is a Y value measured with the two-degree field ofview (C light source) of JIS Z 8701 and subjected to visibilitycorrection, and may be measured with, for example, a spectrophotometer(manufactured by JASCO Corporation, V7100). The polarization degree ofthe polarizer is preferably 99.8% or more, more preferably 99.9% ormore, still more preferably 99.95% or more.

The thickness of the polarizer may be set to any appropriate value. Thethickness is typically from 1 μm to 80 μm, preferably from 3 μm to 40μm.

The polarizer may be typically obtained by subjecting the resin film totreatments, such as a swelling treatment, a stretching treatment, adyeing treatment with the dichromatic substance, a cross-linkingtreatment, a washing treatment, and a drying treatment. The number oftimes of each of the treatments, the order in which the treatments areperformed, the timings of the treatments, and the like may appropriatelybe set. When the resin film is subjected to each of the treatments, thefilm may be a resin layer formed on a substrate.

The cross-linking treatment is performed by, for example, bringing aboric acid solution (e.g., an aqueous solution of boric acid) intocontact with the resin film. In addition, when a wet stretching systemis adopted in the stretching treatment, the stretching is preferablyperformed while a boric acid solution is brought into contact with theresin film. In ordinary cases, the resin film is uniaxially stretched atfrom 3 times to 7 times from the viewpoint that excellent polarizationcharacteristics are obtained. A stretching direction in the stretchingtreatment may correspond to the absorption axis direction of thepolarizer to be obtained. The transmission axis direction thereof may beperpendicular to the absorption axis direction. In one embodiment, whilean elongated resin film is conveyed in its lengthwise direction, thefilm is stretched in the conveying direction (machine direction: MD). Inthis case, the absorption axis direction of the polarizer to be obtainedmay be the lengthwise direction (MD), and the transmission axisdirection thereof may be a widthwise direction (transverse direction:TD).

A-2. Protective Film

As the formation materials of the protective film, there are given, forexample, a cellulose-based resin, such as diacetyl cellulose ortriacetyl cellulose (TAC), a (meth)acrylic resin, a cycloolefin-basedresin, an olefin-based resin, such as polypropylene, an ester-basedresin, such as a polyethylene terephthalate-based resin, apolyamide-based resin, a polycarbonate-based resin, and copolymer resinsthereof. The term “(meth)acrylic resin” refers to an acrylic resinand/or a methacrylic resin.

The thickness of the protective film is preferably from 10 μm to 200 μm.A surface-treated layer may be formed on one side of the protective film(side on which the polarizer is not arranged). Specifically, the sidemay be subjected to a hard coat treatment, an antireflection treatment,or a treatment intended for diffusion or anti-glaring. In addition, theprotective film may function as a retardation film. When the protectivefilms are arranged on both sides of the polarizer like the illustratedexample, the constructions (including a formation material and athickness) of both the films may be identical to each other, or may bedifferent from each other.

A-3. Others

Any appropriate adhesive may be adopted as an adhesive to be used in thebonding of the protective film. For example, an aqueous adhesive, asolvent-based adhesive, or an active energy ray-curable adhesive isused. An adhesive containing a PVA-based resin is preferably used as theaqueous adhesive.

B. Shrinkage

As described above, the method includes shrinking the polarizing filmlaminate. The shrinkage of the polarizing film laminate can provide apolarizing plate excellent in durability. Specifically, a shrunkpolarizing plate shows an extremely small change in shape due to achange in external environment, and hence when the polarizing plate isbonded to any other member (e.g., the glass substrate of a liquidcrystal cell or the like) through intermediation of a pressure-sensitiveadhesive layer, an influence on the adjacent pressure-sensitive adhesivelayer is extremely small. Accordingly, a change in shape of thepressure-sensitive adhesive layer due to the change in externalenvironment is suppressed, and hence the occurrence of a stress betweenthe respective members (e.g., a stress produced when the modulus ofelasticity of the pressure-sensitive adhesive layer increases at lowtemperature) can be prevented. As a result, a crack does not occur inthe polarizing plate and hence the polarizing plate can have extremelyexcellent durability.

A method for the shrinkage is typically, for example, a method involvingheating the polarizing film laminate. A heating temperature is, forexample, from 50° C. to 120° C., preferably from 70° C. to 90° C. Whenthe temperature falls within such range, the polarizing film laminatecan be efficiently shrunk while its optical characteristics (e.g., ahue, a transmittance, and a polarization degree) are secured. A heatingtime is, for example, from 1 hour to 100 hours, preferably 2 hours ormore, more preferably 10 hours or more. The heating may be performed inone stage, or maybe performed in a plurality of stages. In addition, theheating temperature may be kept substantially constant, or may bechanged continuously or in a stepwise manner.

A shrinkage ratio is preferably 0.2% or more, more preferably 0.3% ormore in, for example, the transmission axis direction of the polarizerin the polarizing film laminate. Meanwhile, the shrinkage ratio in thetransmission axis direction is, for example, 0.6% or less. With suchshrinkage ratio, it can be judged that the polarizing film laminate isshrunk to a sufficient level. The polarizing film laminate may shrink inits absorption axis direction to a larger extent than in thetransmission axis direction, and hence at the initial stage of theshrinkage, a dimension in the transmission axis direction of thepolarizing film laminate apparently increases for the time being in somecases. In any such case, as the shrinkage progresses, the dimension inthe transmission axis direction may reduce from a dimension at the timeof the initiation of the shrinkage (at the time of the initiation of theheating).

A shrinkage ratio in the absorption axis direction of the polarizingfilm laminate is preferably 0.3% or more, more preferably 0.4% or more.Meanwhile, the shrinkage ratio in the absorption axis direction is, forexample, 1.0% or less. The shrinkage ratio may be determined from thefollowing equation.

Shrinkage ratio (%)={1−(dimension after heating/dimension beforeheating)}×100

C. Cutting

The polarizing plate of the present invention can be formed into adesired shape because the polarizing plate has excellent durability. Amethod of forming the polarizing plate into the desired shape istypically, for example, a method involving cutting (punching) thepolarizing film laminate. The cutting may be performed before theshrinkage of the polarizing film laminate, or may be performed after theshrinkage of the polarizing film laminate. Excellent durability can beobtained irrespective of whether the cutting is performed before theshrinkage or performed after the shrinkage. The cutting is preferablyperformed after the shrinkage from the viewpoint that the forming intothe desired shape is performed more accurately.

Any appropriate method may be adopted as a cutting (punching) method.For example, a method involving irradiating the laminate with laserlight or a method involving using a cutting blade (punching die), suchas a Thomson blade or a pinnacle blade, is given. The laser lightirradiation provides a smooth cut surface and can suppress theoccurrence of the starting point of a crack (initial crack), and hencecan contribute to a further improvement in durability. Even when thecutting blade is used (even when the initial crack occurs), theshrinkage can provide excellent durability.

Any appropriate laser may be adopted as the laser as long as thepolarizing film laminate (polarizing plate) can be cut. A laser that canemit light having a wavelength in the range of from 150 nm to 11 μm ispreferably used. Specific examples thereof include a gas laser, such asa CO₂ laser, a solid laser, such as an YAG laser, and a semiconductorlaser. Of those, a CO₂ laser is preferably used.

A condition for the laser light irradiation may be set to anyappropriate condition depending on, for example, the laser to be used.When the CO₂ laser is used, an output condition is preferably from 10 Wto 1,000 W, more preferably from 100 W to 400 W.

D. Polarizing Plate

FIG. 2 is a plan view of a polarizing plate according to one embodimentof the present invention. A polarizing plate 100 is suitably used in themeter panel of an automobile. The polarizing plate 100 includes a firstdisplay portion 50 and a second display portion 60 that are continuouslyarranged, and through-holes 51 and 61 for fixing various meter needlesare formed around the centers of the respective display portions. Thediameter of each of the through-holes is, for example, from 0.5 mm to100 mm. The outer edge of each of the display portions 50 and 60 isformed into an arc shape along the rotational direction of a meterneedle.

When a through-hole is formed like the illustrated example, the positionof the through-hole may appropriately be set depending on, for example,the applications of the polarizing plate. The crack is liable to occurfrom the peripheral edge of the through-hole serving as a startingpoint, and the tendency may be more remarkable as the position of thethrough-hole becomes more distant from the outer edge of the polarizingplate. As a result, as the position of the through-hole becomes moredistant from the outer edge of the polarizing plate (e.g., its distancefrom the outer edge of the polarizing plate is 15 mm or more), adurability-improving effect exhibited by the shrinkage can be moresignificantly obtained. Similarly to the peripheral edge of thethrough-hole, a site whose outer edge forms a V-shape (including anR-shape) that is convex inward in a plane direction, such as a boundaryportion 41 or 42 between the respective display portions, is also liableto serve as the starting point of the crack.

The polarizing plate of the present invention is not limited to theconstruction of the illustrated example and may be appropriatelychanged. For example, the shape of the polarizing plate, the presence orabsence of the through-holes, the shapes and sizes of the through-holes,and the number and formation positions of the through-holes mayappropriately be changed.

The polarizing plate of the present invention is bonded to any othermember (e.g., the glass substrate of a liquid crystal cell or the like)through intermediation of, for example, a pressure-sensitive adhesivelayer. The thickness of the pressure-sensitive adhesive layer ispreferably from 4 μm to 50 μm. An acrylic pressure-sensitive adhesive ispreferably used as a pressure-sensitive adhesive forming thepressure-sensitive adhesive layer.

Hereinafter, the present invention is specifically described by way ofExamples. However, the present invention is not limited to theseExamples. A dimensional change ratio is a value calculated from thefollowing equation.

Dimensional change ratio (%)={(dimension after heating/dimension beforeheating)−1}×100

EXAMPLE 1 (Production of Polarizing Film Laminate Sheet)

A film (thickness: 28 μm) obtained by incorporating iodine into anelongated PVA-based resin film and uniaxially stretching the film in itslengthwise direction (MD) was used as a polarizer.

A PVA-based adhesive was applied to one side of the polarizer so thatits thickness after drying became 100 nm, and an elongated TAC filmhaving a thickness of 40 μm was bonded to the polarizer so that theirlengthwise directions were aligned with each other.

Subsequently, a PVA-based adhesive was applied to the other side of thepolarizer so that its thickness after drying became 100 nm, and anelongated acrylic film having a thickness of 30 μm was bonded to thepolarizer so that their lengthwise directions were aligned with eachother.

Thus, a polarizing film laminate sheet having a construction “TACfilm/polarizer/acrylic film” was obtained.

The resultant polarizing film laminate sheet was cut with a CO₂ laser(wavelength: 9.35 μm, output: 150 W) to provide a cut piece of a sizemeasuring 112 mm by 112 mm, the cut piece having a through-hole having adiameter of 2 mm formed in a site distant from its outer edge by 55 mm.

The resultant cut piece was placed under an atmosphere at 85° C. for 50hours to provide a polarizing plate. The polarizing plate had adimensional change ratio of −0.74% (shrinkage ratio of 0.74%) in itsabsorption axis direction and a dimensional change ratio of −0.44%(shrinkage ratio of 0.44%) in its transmission axis direction, thedimensional change ratios each serving as a ratio of a dimension afterthe heating to that before the heating. The dimensional change ratioseach serving as a ratio of a dimension after the heating to that beforethe heating were each determined by: separately preparing a cut piececut out of the polarizing film laminate sheet into a size measuring 100mm by 100 mm (no through-hole was formed in the cut piece); andmeasuring the position of a corner of the cut piece. In this case, thecut piece was cut out of the sheet so that a pair of sides opposite toeach other corresponded to the transmission axis direction of thepolarizer and another pair of sides opposite to each other correspondedto the absorption axis direction of the polarizer.

EXAMPLE 2

A polarizing plate was obtained in the same manner as in Example 1except that the resultant cut piece was placed under an atmosphere at85° C. for 5 hours. The polarizing plate had a dimensional change ratioof −0.45% (shrinkage ratio of 0.45%) in its absorption axis directionand a dimensional change ratio of −0.37% (shrinkage ratio of 0.37%) inits transmission axis direction, the dimensional change ratios eachserving as a ratio of a dimension after the heating to that before theheating, and each being measured by the same method as that of Example1.

EXAMPLE 3

A polarizing plate was obtained in the same manner as in Example 1except that the resultant cut piece was placed under an atmosphere at85° C. for 2.5 hours. The polarizing plate had a dimensional changeratio of −0.34% (shrinkage ratio of 0.34%) in its absorption axisdirection and a dimensional change ratio of −0.25% (shrinkage ratio of0.25%) in its transmission axis direction, the dimensional change ratioseach serving as a ratio of a dimension after the heating to that beforethe heating, and each being measured by the same method as that ofExample 1.

EXAMPLE 4

A polarizing plate was obtained in the same manner as in Example 1except that: the size of the cut piece was set to 52 mm by 52 mm; andthe through-hole was formed in a site distant from the outer edge of thecut piece by 25 mm.

COMPARATIVE EXAMPLE 1

A polarizing plate was obtained in the same manner as in Example 1except that the cut piece was not heated.

COMPARATIVE EXAMPLE 2

A polarizing plate was obtained in the same manner as in Example 4except that the cut piece was not heated.

The durability of the resultant polarizing plate was evaluated by a heatcycle test (also referred to as heat shock (HS) test). Specifically, atest sample was obtained by bonding the resultant polarizing plate to aglass plate with an acrylic pressure-sensitive adhesive (thickness: 20μm). The sample was left to stand under an environment at −40° C. for 30minutes and then left to stand under an environment at 85° C. for 30minutes. The foregoing operation was defined as one cycle and the cyclewas repeated 100 times. After that, whether or not a crack occurred inthe polarizing plate was observed.

FIG. 3A to FIG. 3D are photographs obtained by observing the peripheriesof the through-holes of the polarizing plates of Examples 1 to 3 andComparative Example 1 after the HS tests with an optical microscope(manufactured by Olympus Corporation, MX61, magnification: 5). InComparative Example 1 (FIG. 3D), a crack that can be visually recognizedwith the eyes in a clear manner is observed. In contrast, in Example 1(FIG. 3A), the occurrence of a crack (including a microcrack) is notobserved. In each of Examples 2 and 3 (FIG. 3B and FIG. 3C), amicrocrack that cannot be visually recognized with the eyes in a clearmanner is observed, but the occurrence of a crack is suppressed ascompared to Comparative Example 1. The cracks each occur along astretching direction.

In Example 4, as in Example 1, the occurrence of a crack (including amicrocrack) is not observed. In Comparative Example 1, the crack extendsfrom the through-hole serving as a starting point to an end side of thepolarizing plate. In contrast, in Comparative Example 2, a crack lengthis 12 mm.

FIG. 4A and FIG. 4B are each a photograph for showing the state of anend portion of the polarizing plate of the test sample of Example 1after the HS test, and FIG. 5A and FIG. 5B are each a photograph forshowing the state of an end portion of the polarizing plate of the testsample of Comparative Example 1 after the HS test. In ComparativeExample 1, a region in which the pressure-sensitive adhesive layer usedat the time of the bonding of the polarizing plate to the glass plate isexposed is formed.

The polarizing plate of the present invention can be suitably used notonly in an image display apparatus (a liquid crystal display apparatusor an organic EL device) of a rectangular shape but also in, forexample, an image display portion of a particular shape typified by themeter display portion of an automobile or a smart watch.

Many other modifications will be apparent to and be readily practiced bythose skilled in the art without departing from the scope and spirit ofthe invention. It should therefore be understood that the scope of theappended claims is not intended to be limited by the details of thedescription but should rather be broadly construed.

What is claimed is:
 1. A method of producing a polarizing plate,comprising: preparing a polarizing film laminate including a polarizerand a protective film arranged on at least one side of the polarizer;shrinking the polarizing film laminate; and then cutting the shrunkpolarizing film laminate.